Vectors containing the entire rat growth hormone gene have been transfected into GH3 x L cell, non-growth hormone-expressing somatic cell hybrids. Growth hormone expression is thereby reestablished in the hybrids, and in some is under glucocorticoid control. Human glucocorticoid receptor, covalently labelled with 3H dexamethasone mesylate has been characterized by physical methods and by its' reactions with anti-human glucocorticoid receptor antiserum. Rat and human glucocorticoid receptors have been purified more thoroughly. Somatic cell hybrids have been made to analyze cells bearing various mutations in the human glucocorticoid receptor. 3H cortivazol binds to sites in wild-type CEM cells; one appears to be the glucocorticoid receptor. Hybrid genes between the promoter region the rat growth hormone gene and the E. coli galactose kinase gene have prepared, put in vectors and transfected into various cells.